I have never heard of anyone offering to make custom coaches. I have done a lot of models based on existing commercial products, such as new laser-cut styrene sides for Accucraft Jackson and Sharp Fn3 coaches. I also know of companies who will cut coach sides and ends from wood. Both types require that you supply drawings.
New sides for an existing coach is not a cheap project, as you have to find a donor coach. Mine ended up costing me around $500 each. A full custom build is probably more like $1000-2000.
As I suspect you want a 1/32nd scale std gauge car, take a look at Smoke'm and Stoke'm. http://www.livesteamg1us.com/cars.html
Bob Clark had the cars made by IHE for the Bowande G5. They may be close to your requested types. IHE may be willing to undertake a custom version, as they already have the basics.